1. Field of the Invention
The present invention relates to a method of forming outer leads of electronic parts such as resin molded semiconductor devices, and QFPs (Quad Flat Packages) and SOPs (Small Outline Packages) including at least one semiconductor chip, and a lead forming machine therefor.
2. Description of the Related Art
For mounting electronic parts, such as resin molded semiconductor devices, and QFPs and SOPs including a semiconductor chip, on a circuit board such as printed circuit boards, after solder pellets are previously provided on the board by a printer, outer leads of the electronic parts are in contact with the solder pellets. In this state, the circuit board is supplied to a heating furnace to carry out soldering of the electronic parts by a reflow system.
As shown in FIG. 1A, the surface mounting type electronic part 101 includes at least one outer lead 102 or 103 almost horizontally extending from both side surfaces of the resin layer (or package) thereof. As shown in FIG. 1B, these leads include flat portions 102a and 103a with a predetermined length located near the roots thereof, and flat portions 102b and 103b to be soldered to the circuit board. For example, the flat portions 102a and 102b are provided by clockwise bending the outer lead 102, extending from the side of the resin layer (or package) of the electronic part 101, to approximately 90.degree. except for a part of the root, and by counterclockwise bending a part of the bent outer lead to approximately 90.degree. .
FIG. 2 is a sectional view showing a main part of a conventional lead forming machine 200. The forming machine 200 comprises a fixed unit held by a die holder 203 and including a bending die 202 fixed on a bolster 201, a semi-movable unit including a stripper 204 held by a stripper holder 205, and a movable unit which includes first and second plate-like bending punches 206 and 207, which can be vertically moved through a perforation provided in the stripper 204, thereby forming leads 102 and 103 of the electronic part 101 arranged in a space rendered by the bending die 202 and the stripper 204, a punch holder 208 and a spacer 210 holding these bending punches and fixed on a backup plate 213, a support block 209 attached to the spacer and arranged to have a predetermined distance from the upper surface of the stripper 204, and an upper holder 214 for supporting the backup plate 213 and holding the stripper holder 205 through a spring member 212 having a high elastic force.
The bending die 202 and the stripper 204 for placing the electronic part 101 have first projections 202a and 202b and second projections 204a and 204b which are faced each other and clamp the leads 102 and 103 near their roots. In a state wherein the lead formation is completed, the lower surface of the support block 209 contacts the upper surface of the stripper 204 to control a movement of the first and second bending punches 206 and 207. The lead forming machine 200 is provided with an alignment pin 215 for setting the electronic part 101 at a predetermined position in the bending die and stripper, and a knockout pin 216 for taking out the electronic part from the predetermined position. In addition, stopper plates 211 are provided to the first and second bending punches 206 and 207.
Using the lead forming machine 200 shown in FIG. 2, a process for forming the outer lead 102 of the electronic part 101 will be described below with reference to FIGS. 3A to 3C. (Note that the outer lead 103 is omitted for the sake of illustrative convenience).
As shown in FIG. 3A, when the movable unit having the first and second bending punches 206 and 207 is moved downward, the lead 102 of the electronic part 101 arranged in the space between the bending die 202 and the stripper 204 is strongly clamped between the first projections 202a and 204a, which are faced each other near the root of the lead 102, by a high reaction force of the spring member 202 located between the stripper holder 205 and the upper holder 214.
As shown in FIG. 3B, when the movable unit is further moved downward, the clamped lead 102 is clockwise bent to approximately 90.degree. by the first bending punch 206 to form a flat portion 102a near the root thereof, thereby performing primary lead forming.
As shown in FIG. 3C, when the movable unit is further moved downward, the lead 102 is counterclockwise bend to approximately 90.degree. by the first bending punch 206 to form a flat portion 102b of the lead, thereby performing secondary lead forming. Thus, the conventional lead forming process is completed.
As shown in FIG. 4A, however, in the outer leads 102 and 103 horizontally extending from both side surfaces of the resin-molded electronic part 101, its coplanarity is not always uniform because unwanted flashes or burrs are caused on the resin layer during the resin molding, the caused flashes are removed therefrom, or the resin layer expands or contracts due to heat. Therefore, unwanted deformation will be caused in the outer leads. That is, in FIG. 4A, the lead 102 which is bent to a right-upward direction and the lead 103 which is bend to a left-downward direction are shown. In addition, in the conventional lead forming, the primary lead forming is performed while the roots of the outer leads 102 and 103 are strongly clamped, and the flat portions 102b and 103b to be soldered to the circuit board are subsequently formed in the secondary lead forming. Therefore, since the deformation of the flat portions 102a and 103a especially near its roots will not be changed before and after the forming process, it cannot be controlled.
That is, as shown in FIG. 4B, since "spring back" of each lead occurs within the elastic deformation, the unwanted deformation of the outer leads prior to the lead forming process is still remained, so that the coplanarity of the leads to be soldered is not improved. Accordingly, a highly reliable lead forming cannot be provided.